It’s the multiple volumes that are throwing it.
You want to mount the drive at /media/HDD1:/media
or something like that and configure Radarr to use /media/movies
and /media/downloads
as it’s storage locations.
Hardlinks only work on the same volume, which technically they are, but the environment inside the container has no way of knowing that.
Hugo can be as simple as installing it, configuring a site with some yaml that points at a really available theme and writing your markdown content.
It gets admittedly more complex if you’re wanting to write your own theme though.
But I think this realistically applies to most all static site generators.